Relations among motions of the reticular lamina (RL), tips of hair bundles, and endolymphatic surface of the tectorial membrane (TM) in the tectorial region of the excised cochlea of the alligator lizard (Gerrhonotus multicarinatus) were investigated. Sound stimuli from 105-140 dB SPL measured in the fluid adjacent to the basilar membrane, with frequencies from 40 Hz to 4 kHz, were used to excite cochleae in eight preparations. Images taken with a video microscopy system were analyzed to determine three-dimensional motions of cochlear structures. Motions of these structures were generally broadly tuned in frequency, with peak amplitudes smaller than 1 micrometer peak-to-peak.
Three micromechanical relations were investigated as functions of frequency: (1) TM/RL, the motion of the endolymphatic surface of the TM normalized to RL motion; (2) (TM-RL)/RL, the shear between the endolymphatic surface of the TM and the RL, normalized to RL motion; (3) (Tips-RL)/(TM-RL), the ratio of the deflection of the tips of hair bundles relative to the RL to shear between the endolymphatic surface of the TM and the RL. Although there was considerable variation across preparations, two trends were evident. First, the magnitude of TM/RL was less than 2. Second, none of the micromechanical relations showed sharp frequency selectivity.
The measured magnitudes of TM/RL are small compared to those predicted by resonant TM models of cochlear micromechanics. This difference suggests that TM resonance does not significantly contribute to the frequency selectivity of the lizard cochlea. The frequency selectivity of the measured micromechanical relations is broad compared to that seen in responses of auditory nerve fibers innervating the TM region. Thus, mechanisms other than mechanical resonance may contribute to frequency selectivity in the tectorial region of the alligator lizard cochlea.